A couple years ago I was toying around with libjit in python, wrote some bindings, traced down (most of) the seqfaults and then got distracted by other things. Was going to dust it off to try to hook into pep-0523 but totally forgot about it until you posted that link.
What is your impression of libjit? It looks really nice. I've only tried GNU Lightning, and I made Python bindings for it (although, today I would have used cffi to interface with it instead of my ctypes approach, because of the header files): https://github.com/cslarsen/lyn
I'd love to see a comparison of the various JIT libraries. I guess both give you optimizations and register allocation for free. Of course, the JITting speed is quite important as well.
It seemed pretty solid though I didn't do anything too complicated with it.
Mostly what messing around with it did was taught me that I had a lot to learn about compiler construction which is what I've been slowly doing since then.
https://github.com/eponymous/libjit-python